1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system and a magnetic resonance imaging method, which are used for medical purposes, and in particular, to a system and method enabling fast and/or wide-range imaging with a couch (tabletop) on which a patient (object to be imaged) is laid.
2. Description of Related Art
Magnetic resonance imaging is a technique for magnetically exciting nuclear spins in an object located in a static magnetic field by applying a radio-frequency (RF) signal with the Larmor frequency, and reconstructing an image of the object using an FID (free induction decay) signal or echo signal induced by the excitation. In the field of magnetic resonance imaging, like imaging using other modalities, a wide variety of imaging techniques have been developed thanks to recent advancements in hardware.
For example, for imaging blood vessels in the inferior limb, various imaging techniques are already known. Normally, imaging the inferior limb requires an imaged region to be wider in the body axis direction. Further, it is also required that imaging in such a wider range be finished during an interval of time during which a contrast agent remains in the inferior limb. Hence imaging should be done at a higher speed (in a shorter time) and in a higher temporal resolution.
One technique, which is able to have a wider imaging range, is an imaging technique in which the patient is moved on a couch, called a moving bed technique, by which the imaging is done with a patient's couch moved. When this technique is used, the first scan for a given region is conducted at a certain patient's couch location, the couch (patient) is moved by a distance corresponding to the given region, and then a second scan is conducted for the next region to be imaged. This combination of scans and couch movements is repeated in turn, so that a desired region to be imaged, such as the inferior limb, is entirely covered. After the imaging, the obtained images are lined up in, for example, the body axis direction or combined into a single image to be used for diagnosis.
The moving bed technique includes a first technique that uses a whole-body coil with the couch moved and a second technique that uses multiple RF coils.
In the case of the technique involving the whole-body coil, the whole-body coil is fixed to a magnet. The couch is moved stepwise by a distance equal to a predetermined region to be imaged, and after each movement to a new region to be imaged, imaging is done.
FIG. 1 exemplifies a magnetic resonance imaging system capable of performing the moving bed technique by which a whole-body coil 101 is used. In the figure, reference 102 is a magnet that generates a static magnetic field. Connected to the whole-body coil 101 through a duplexer 103 are a transmitter 104 and a preamplifier 105. The preamplifier 105 is connected to a receiving-system circuit 106, both of which make up a receiver. A host computer 107 responsible for control of the whole system is placed to control a transmitter 104 and a gradient amplifier 109 via a sequencer 108, so that a given pulse sequence is performed. Connected to the host computer 107 are an input device 109, display 110, and storage 111. The host computer 107 controls a couch driver (not-shown) to move a tabletop 112 of the couch based on the moving bed technique.
On the other hand, when the second technique is used with multiple RF coils, the multiple RF coils themselves are fixed to an object or the couch, so the coil can be moved with the couch. The multiple RF coils consist of, for example, a plurality of sets of coil members (a group of coils) disposed in an array. In this case, in response to movement of the couch under imaging, each set of coil members enters a uniform static field region within a magnet's bore, and then exists therefrom.
When it is desired to use the multiple RF coils based on the moving bed technique, it is conceivable that a plurality of images obtained through a plurality of sets of coil elements are made to cover all of a desired region to be imaged of an object.
FIG. 2 exemplifies a magnetic resonance imaging system that is able to execute the moving bed technique with such multiple RF coils. The multiple RF coils 121 consist of three sets of coil members (coil groups) 1 to 3. Each set of the coil members 1 to 3 is independently routed to a host computer 107 through a preamplifier 105a (to 105c) and a receiving-system circuit 106a (to 106c), respectively. That is, the multi RF coil 121 and the circuitry to receiver and process detected signals of the coil are added to the constitution shown in FIG. 1.
Meanwhile, to meet the foregoing requirements of shortening the imaging time, higher-performance technologies in the hardware, as in improvement of a booster technology to shorten a switchover time of gradients, are now under development.
As another imaging technique, high-temporal-resolution dynamic imaging is known for observing the passage of a contrast agent at high temporal resolution. This imaging includes a key-hole imaging technique (for example, refer to “R. A. Jones at al., “Dynamic, contrast enhanced, NMR perfusion imaging of regional cerebral ischaemia in rats using k space substitution” SMRM 1992, p. 1138) and a view share technique (for example, refer to U.S. Pat. No. 4,830,012). When the key-hole imaging technique is used, data for one image is acquired first, and then dynamic imaging is done, during which time only data mapped in a central part of the k-space used for image reconstruction are updated. Meanwhile, the view share technique, which abandons the procedures of updating all the k-space with new data before reconstructing the k-space into an new image, uses the k-space previously divided in a plurality of regions. An image is reconstructed whenever a plurality of divided regions of the k-space are partly replaced with new acquired data, thus raising an image update rate. Another known imaging technique is called 3D-TRICKS (for example, U.S. Pat. No. 5,713,358, which is provided by improving the key-hole imaging technique to 3D MRA. The central part of the k-space is therefore raised in the data update rate with respect to the remaining region, so that the key-hole imaging technique is effective in observing the passage of a contrast agent at higher temporal resolution than that of the normal view share technique.
However, the above various types of conventional techniques have some drawbacks. For using the moving bed technique that involves the multiple RF coils to widen a region to be imaged, it is required that a plurality of sets of coil elements be switched over to select one set located at the uniform static field region whenever the couch is moved by an amount corresponding to the length of a region to be imaged. In order to realize this, as an operator observes the couch that is under stepwise movement, the operator must manually switch over one set of coil elements to another set. This complicates operations and lacks accuracy, in addition to taking much time.
Further, the current hardware techniques are still lacking when realizing both of the shortened imaging time and the higher temporal resolution. The foregoing key-hole imaging and other imaging techniques are still short of temporal resolution. In addition, the foregoing imaging techniques such as the key-hole imaging is simply raising apparent temporal resolution by updating data mapped in part of the k-space, thus lacking a depiction performance of minute structures. Moreover, the foregoing imaging techniques such as the key-hole imaging can be applied only to a situation that a region to be imaged of an object is constant and changes of intensity at part of the region to be imaged, which is due to a contrast agent, are observed. But those techniques are unavailable for imaging that requires the couch to be moved such that different regions are sequentially subject to imaging at a high speed.
Recently, concerning this fast imaging, a technique of gaining a shortened imaging time by using multiple RF coils has been spotlighted (for example, refer to “10th” Ann. Scientific Meeting SMRM 1240 (1991),” which is effective in speeding up approximately all types of magnetic resonance imaging techniques. This technique enables imaging to be performed with using less encoding steps than necessary for reconstructing a single image. The resultant folded data (i.e., aliasing) is dissolved based on the fact that a plurality of coil elements constituting the multiple RF coils are different in their sensitivity distributions from each other, so that images with no folded data are obtained. Since this fast imaging allows the number of encoding steps to be lessened in proportion to the number of coil elements, unlike the ordinal imaging methods, the imaging time can be shortened.
However, the moving bed technique involving a single whole-body coil cannot be applied to such fast imaging. Further, in cases the moving bed technique involving multiple RF coils is applied to the fast imaging, it is necessary that an operator switch over a plurality of sets of coil elements, as described before. This means that the fast imaging cannot provide its merits satisfactorily.